By default any application capable of showing / emitting notifications should have them all turned off by default. All of them. Do not make users wonder, "Where did that sound come from?" "What was that pop-up that just appeared and disappeared in the corner of my monitor?" I'm sure some folks want, for example, their terminal's "visual bell" to emit a sound or pop up a dialog. They can open the preferences and enable it if so. It's not something that users should have to hunt down and turn off. Application developers: remember that your application is probably one of 20 that is running on somebody's desktop. It needs to coexist nicely, not scream for attention. Otherwise we have 20 applications that need their notifications smacked down so people can get work done without distractions.